Of all the synthetic polymers considered as materials with useful gas permeation properties, such as resistance to passage of oxygen, carbon dioxide, water, and the like, poly(vinyl alcohol) (PVOH), which is a polymer made up of units of the structure ##STR1## and generally prepared by the total hydrolysis of homopolymers of vinyl acetate or related vinyl esters, the starting polymer made up of units of the structure ##STR2## where R is alkyl, that is, from one to eight carbon atoms, preferably methyl, ranks as the most impervious to the passage of small molecules. PVOH derives this property from the high cohesive energy density and polarity of the hydroxyl groups. The presence of the network of hydroxyl groups has the concomitant effect of rendering the polymer (PVOH) impermeable to gases, but sensitive to moisture. The strong intermolecular interaction resulting from the high polarity of the --OH functional group gives rise to a melting temperature in the vicinity of the degradation temperature of PVOH. Consequently, melting is accompanied by degradation. The degradation is so severe that PVOH by itself cannot either be melt extruded or injection molded. Co-polymers having a low mol percentage of ethylene, such as from about 5 to about 25 mol percent, are similar to poly(vinyl alcohol) in that they cannot be melt-processed into film without the aid of plasticizers.
In U.S. Pat. No. 5,189,097, filed Oct. 22, 1991, which has some of the same inventors and the same assignment as the present application, are disclosed additive polymers useful in allowing melt processing of the poly(vinyl alcohol) materials discussed above without significant alteration of their physical and barrier properties. These additive polymers are copolymers of lower alkyl methacrylates with a variety of nitrogenous monomers, especially those bearing amide groups, and most especially N-vinylpyrrolidone. Further is disclosed as more useful additives terpolymers containing lower alkyl methacrylates, the same nitrogenous co-monomers, and copolymerized unsaturated carboxylic acids, such as methacrylic acid. It is further disclosed that these latter terpolymers form segmented copolymers on blending with the poly(vinyl alcohol) matrix polymers.
In a patent application in the United States, Ser. No. 872,478, filed on Apr. 23, 1992, now abandoned for a continuation-in-part application, Ser. No. 07/988,548, filed Dec. 10, 19992, now U.S. Pat. No. 5,378,759 also with some of the same inventors and the same assignment as the present application, is disclosed that copolymers of lower alkyl methacrylates with unsaturated carboxylic acids, such as copolymers of methyl methacrylate with methacrylic acid, are also useful for the same purposes.
What has been discovered in the present invention is that these blends of poly(vinyl alcohol) with lower alkyl methacrylate copolymers, with at least one of copolymerized acid or amide, especially cyclic amide, functionality, when co-extruded with structural polymers, produce co-laminates of excellent physical, optical and barrier properties, which do not need additional adhesives to prevent delamination, and which further lend themselves readily to recovery of the structural polymer components of the laminate in a purified form which allows ready recycling, by such means as direct re-use of the film or sheet (if its integrity has been maintained), or by re-processing of comminuted powder, or conversion of the structural polymer into "monomeric" fragments which can be re-polymerized into useful polymers.
In the applications noted above, Applicants had found that a polymeric blend comprising from about 50 to about 90 parts of a first polymer containing at least 50 mol % of units of the structure ##STR3## and optionally units of the structure where R is H or --CH.sub.3, and from 10 to about 50 parts by weight of a polymer containing from about 70 to about 98 parts of units derived from a lower alkyl methacrylate, and at least one of either up to about 30 parts of units derived from a vinyl or vinylidene monomer containing a cyclic amide group of units of the structure ##STR4## where n is 2, 3, 4, or 5, or up to about 30 parts of units derived from an unsaturated carboxylic acid or anhydride, may be melt-processed into useful objects such as sheet, film, and fiber. Applicants further had found that blending of the two polymers by melt-mixing will form a segmented melt-processable polymer wherein the two polymers are chemically combined to form a graft copolymer. Applicants had further described the co-extrusion of the blended polymer with another film-forming polymer to form a co-laminate, and the ability to separate the laminate by contacting with water.
The present invention relates to the specific combination of these and related blends with certain structural polymers to form a multilayer composite structure by co-extrusion or other process where the poly(vinyl alcohol) is melt-processed into a film or sheet, the multilayer laminates having good barrier properties and other physical properties attractive for processing and packaging, and to a process for readily recovering the structural polymers in a form from which various recycling routes may be chosen, depending on the nature of the structural polymer. Since an ability to recycle plastic waste is critical to solution of our waste management problems, and since purity of the recyclable waste stream, whether to reprocess into packaging material, to process for other plastics uses, or to depolymerize the polymer into useful small molecules, is critical to recycling, the present invention offers a convenient method, which further uses poly(vinyl alcohol), a polymer of outstanding barrier properties and adhesion in multi-layer structures.